Grafts of Japanese quail limb and bone rudiments onto the choriallantoic membranes (CAM) of chick embryos are being used to study the linkage of skeletal cells. Such tracing is possible because the morphology of the quail interphase nucleus is distinctly different from that of the chick. Skeletal cell lineage has been evaluated in (1) bone rudiments transplanted before and after the establishment of the marrow cavity, (2) bone rudiments fractured several days after transplantation, and (3) grafted pieces of perichondrium-free epiphyseal cartilage. These experiments have shown that while osteoblasts are derived from precursors located within the bone rudiment, osteoclasts have their origin from the blood vascular system. In addition, these studies have demonstrated that fractured embryonic bones heal rapidly and without callus formation, and that chondrocytes may survive hypertrophy and redifferentiate into osteoblasts. Experiments have recently been initiated in which the quail-chick chimeric system will be used to establish the first appearance of the osteoclast during embryonic development, and to determine the life span of the osteoclast. The marker system will also be incorporated into experiments designed to identify and isolate the hematopoietic precursor of the osteoclast.